The 007 movies have traditionally opened with a shot of Bond turning to fire at an unseen sniper followed by blood dribbling down the screen.
Daniel Craig's Casino Royale built the sequence into a scene of Bond's first kill, while Skyfall continued the trend set by Quantum of Solace by placing it at the end of the movie.
Speaking at a 'Meet the Filmmaker' Q&A at Apple's Regent Street store, Mendes said Skyfall's opening shot meant he couldn't kick off the film with the iconic gunbarrel shot.
"The gunbarrel is him walking, stopping and lifting a gun. When I put the two together it looked ridiculous - shoot the gun already! We tried and couldn't make it work.
"I won't give away why I think it works very well at the end, but it just worked out that way because of the Bond theme and all that sort of stuff."
Mendes added that he grew up a fan of the traditional Bond film opening, saying: "I remember when I was a kid thinking that it was almost the best bit. Everything is possible in that moment, it's like Christmas Eve!"
Bond franchise producer Barbara Broccoli recently told Digital Spy that she would "love" Mendes to return to helm more Bond outings. The Oscar-winning American Beauty director has refused to completely rule out a return to the series.
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Photo gallery - Skyfall in pictures: